POSTKARTE Tyrrell Leith, E... Author: Tyrrell Leith, E... Recipient: Haeckel, Ernst Dating: 29.08.1887
BRIEF OHNE UMSCHLAG Fürbringer, Bruno Author: Fürbringer, Bruno Recipient: Haeckel, Ernst Dating: o. D.
BRIEF OHNE UMSCHLAG Stobbe, August Author: Stobbe, August Recipient: Haeckel, Ernst Dating: 15.06.1880